This invention relates generally to appliances for heating air. More particularly, the invention relates to an inducer for drawing combustion gases through the heat exchangers of a condensing hot air furnace.
Hot air furnaces are widely used to heat enclosed spaces such as residential and commercial buildings. In such a furnace, a fuel is burned in the presence of air to produce hot gases of combustion. Where the fuel is a fuel gas, the gas is mixed with air to form a combustible mixture which is then burned to produce the gases of combustion. These gases pass through a primary heat exchanger in the furnace before passing through a flue to the atmosphere. Air from the space to be heated, from a source external to the space(s) to be heated or a mixture of air from both also passes through the primary heat exchanger, via a flow path that is separate from but in heat exchange relationship with the flow path of the hot combustion gases. The heated air then passes out of the furnace to be conducted to the space or spaces to be heated via appropriate means such as ducting.
Where the fuel is a gas, there is an optimum proportion of fuel to air in the combustible gas mixture that will not only result in the most efficient combustion of the fuel but also minimize the production of undesirable combustion products such as carbon monoxide and various oxides of nitrogen.
One combustion product of the fuels typically used in hot air furnaces is water. In many prior art furnaces, the gases of combustion pass directly from the primary heat exchanger to the flue. The water vapor in the gases of combustion thus passes out of the furnace and the heat contained in the water vapor is lost. In order to increase efficiency, many hot air furnaces now include heat exchangers for condensing the water and thus recover heat that would otherwise be lost.
Many furnaces depend on natural convection to supply draft to move gases of combustion from the burner or burners through the primary heat exchanger and out the flue. However, natural draft may not be capable of providing sufficient flow through the condensing heat exchanger if one is installed. In addition, improved heat transfer performance can be achieved if the flow of air to be heated is counter to the flow of gases of combustion. This counter flow cannot exist in a furnace in which there is both natural draft and an upflow of the air to be heated. And still further, space and other considerations may require that a particular furnace not be of the updraft type that can use natural draft. In these situations, some type of forced, usually induced, draft is employed to cause flow through the flow path for gases of combustion. In an induced draft furnace, an inducer takes a suction on the condensing heat exchanger, and thus on the primary heat exchanger, and discharges to the flue. The condensing heat exchanger may not be completely effective at condensing all the water vapor in the gases of combustion that flow through it. Therefore, some water vapor may pass through the inducer and into the flue. Some of this vapor may condense in the inducer housing or in the flue. Since the inducer is generally located below the flue, any water that condenses in the flue will drain back into the inducer housing. To prevent the inducer housing from becoming flooded with water, some means must be provided for draining collected water from the housing.
If space and other considerations, including ducting and flue arrangements, permit, the typical furnace installation usually includes an upflow furnace, in which the air to be heated flows upward through the furnace. But such considerations may dictate that a particular furnace installation be of some other type, such as downflow or even have an air flow that is horizontal. Downflow arrangements are typically seen in heating systems for individual units in residential apartment buildings. Furnaces installed in the crawl spaces beneath houses typically have horizontal air flow.
Manufacturers have usually met the demand for furnaces capable of installation in a variety of orientations by designing and building furnaces having configurations specialized to each orientation. This practice results in the necessity to have a variety of different furnace model series with a resultant increase in the amount of inventory required at all levels in the chain of supply.
For simplicity in design and manufacture, it is common to make a given fundamental furnace design in several different heating capacity ranges. This is usually done by varying the number of burners and the associated heat exchanger cells. An increase in the number of burners requires an increase in the rate of flow combustion air into the furnace and in combustion gases through and out of the furnace. The rate of gas flow through an induced draft furnace is a function of the characteristics of the inducer and the pressure losses in the flow path of combustion air and gases of combustion both in the furnace itself and in the flue that serves the furnace. In order to simplify manufacturing and inventory management, it would be desirable to be able to use the same inducer in all furnaces of the same fundamental design, regardless of the orientation in which a given furnace may be installed and regardless of the heating capacity of that furnace.
What is needed is a hot air furnace of a single design that can be installed and operated in a variety of orientations, i.e. in either an upflow, a downflow or a horizontal flow configuration. And, to meet this requirement, the furnace must have an induction system that is capable of operating regardless of the orientation of the furnace in a given installation. The draft of the inducer should be variable so that the proper mix of air and fuel as well as the proper flow of gases of combustion can be achieved in all furnaces, regardless of heating capacity, in the model series.